A minor population of B cells, B1 cells, is notable for distinctive cell surface antigens, early appearance, unusual tissue distribution and a novel repertoire of antibody specificities. These antibodies are frequently autoreactive, but there is no evidence that they are involved in autoimmune disease. Virtually all examples of human B cell CLL, B cell lymphomas in AIDS patients and many murine B cell lymphomas exhibit the B1 phenotype. Two distinct hypotheses have been put forward to explain these different phenotypes. One states that the phenotypes are inherent in separate lineages of B cells (B1 and B2), determined in distinct precursors prior to the rearrangement of Ig genes. The other hypothesis states that newly formed B cells, termed B0 cells, can be induced to differentiate along either of two pathways, termed B1 and B2, depending on the nature of the antigen stimulation. Multivalent antigen, leading to cross linking of surface Ig without involvement of T helper cells, induces the B1 phenotype, whereas cognate interaction with antigen and T helper cell induces the B2 phenotype. The majority of B cells are viewed as being virgin B0 cells that retain the capacity to differentiate along either pathway. This project is designed to resolve the issue by determining whether the B1 phenotype is programmed before or after expression of surface Ig. The model being used is the immune response of mice to pyrrolidone (Py), presented as a thymus independent polymer, or coupled to protein as a thymus dependent antigen. The data generated so far are consistent with the latter hypothesis. One series of experiments remains to be done. A heavy chain from a lambda light chain expressing hybridoma, has been used to make transgenic mice which are being bred to kappa chain deficient mice to yield a transgenic mouse strain that produces large numbers of B cells bearing surface Ig reactive with Py. The phenotype of the cells will be determined both before and after immunization with TI or TD forms of Py. Immunizations will be done in vivo in transgenic mice and after transfer of B cells to compatible recipients, as well as in vitro. Immunization with TD antigen will be supplemented with carrier specific T cells. Results from the project should demonstrate whether the strong neoplastic propensity of B1 cells is an inherent feature of a discrete lineage of cells or results from chronic antigenic stimulation of B0 cells with TI antigens.